Need a bit of a crystal ball for that one! Inflation rates, sterling baht rates, who knows what these will do. If you can invest the lump sum and get some return, even if it's not equivalent to what you need each year, at least that would help a little.
What are your basic lifestyle needs? There's a major difference between the expenditure of a fun-loving socialiser with a taste for the ladies, never cooks for himself, eats regularly in farang restaurants, and who likes a beer or three on a regular basis... and that of a quiet guy who is content with a night or two out each week, eats pretty much nothing but Thai food, and is more into the cultural scene.
The former can get through two or three grand a month in no time, at today's costs. The latter can easily survive on half that.
When I stay in our house in the North East I can live for five US$5 a day for food etc. In Bangkok you can get a reasonable appartment from about $300 a month up. Renting is arguably better than buying, because you find it hard to get more than a 30 year lease and in general you can rent for 18 years for the same cost as buying. Selling a used place is very hard because of the lease and everyone wants a new place so old places go cheaper. If you want to live a fully Western lifestye in Bangkok, Pattaya or Phukket it will be not much cheaper than the USA, which is about half the UK cost of living. Hope this helps, it is based on my own experience and research.
you can get a fairly decent apartment near the entertainment area (sukhumvit soi 11 for THB30k / Month fully furnished). Close by to nana plaza + other entertainement venues)
Transportation is cheap (motorcycle taxis cost 20 baht to the main street sukhumvit) Then you have the sky train which takes u to major entertainment and shopping areas.
Food is fairly cheap if you eat street side (40 baht for plate of fried rice or noodles)
Drinks are about thb120 - 160 / drink or cheaper at nana plaza)
ofcourse u can get cheaper rentals but again it all depends on what u r looking for.
hope that helps